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-   -   Model Engine Spitting Fuel pics (http://www.rcuniverse.com/forum/glow-engines-114/11255347-model-engine-spitting-fuel-pics.html)

earlwb 10-08-2012 10:58 AM

Model Engine Spitting Fuel pics
After many attempts over the years I finally managed to get a decent picture of a model engine spitting fuel out of the carburetor along with the vapor cloud (fuel standoff) that forms at the outer lip on the carburetor too. I was trying out a new digital camera that has a faster shutter speed than my other cameras. I remember years ago trying to do this with a 35mm film camera and not having any luck doing it. These digital cameras have come a long way from being real slow at taking pics to having some good speed to them nowadays. It might have helped in this case with the fuel having a darker blue color from the blue color oil in it too.

All of the engines be it glow, gasoline or diesel tend to spit fuel and form a vapor cloud at the intake to the carb. Some engines do it more than other engines do. It is mainly a function of the port or valve timing setup with a particular engine as to how much they do it. You have probably seen it with some engines before when you are holding your hand behind the engine and feel the tiny cold fuel droplets on your hand as the engine is running.

In this pic the little specs or dots in the pic are the fuel droplets of different sizes reflecting the light from the flash back. Also you can see how the fuel droplets are pulled towards the base of the propeller and then get blown back towards the rear as the droplets of fuel move farther out and catch the windstream off the prop. Now right at the outer lip of the carburetor, you can see the rather ephemeral and ghostly lookingvapor cloud that forms there, it is a little cone shaped and tapered, and at the tip it twists and tapers back as it gets blown back too.



RCPAUL 10-08-2012 11:16 AM

RE: Model Engine Spitting Fuel pics
Nice photos!


Mr Cox 10-08-2012 11:50 AM

RE: Model Engine Spitting Fuel pics
That's nice to see, so one should use a taller intake for better fuel economy.

Funny with the see-through prop too, sharp edges yet see-through. Is that from a red-eye reduction mode?

earlwb 10-08-2012 05:25 PM

RE: Model Engine Spitting Fuel pics

Yeah I found it fascinating in that even the scratch on the spinner was caught and it was turning at about 2,500 RPMs even.
The camera is a Olympus VR-320 which is a 14mp model. They have a 16mp version now also. The settings were all auto in the camera. So it could have used anywhere from a ISO 30 to a ISO 1600 setting when it determined what to use to take the pic. Shutter speed up to 1/2000. They seem to have put in a lot of smarts into the microprocessor in the camera. I hadn't noticed it had a ISO 1600 setting for high sensitivity before. It does also have a setting to allow it to take pics down to as close as .5 CM for closeups too.

I liked how it showed the interesting vortex effect with the propeller spinning. Near the center at the base of the propeller blades the air is drawn forward and then thrust back further out on the blades. The fuel spitting actually has the droplets going forward and some of them get caught on the blades and are slung further out and the thrown off of the blades towards the rear. I saw similar effects on older engines using the fuel to flow through and out of the front of the crankcase on the crankshaft. The fuel/oil would dribble out and get sucked into the blades in the center, flow outwards and get thrown off the blades.

I am going to have to try some more picture taking using the high ISO 1600 setting now to see what other interesting effects I can get.

1QwkSport2.5r 10-08-2012 06:53 PM

RE: Model Engine Spitting Fuel pics
Nice photos, Earl. I have a DSLR camera thats 10.1mp, but has a very sensitive sensor. I may try to see if I can capture the same events you have captured. I dont run engines at night very often, but I suspect its much easier to see the fuel droplets at night with the flash.

The subject has come up before, and there are terms associated to the fuel spit-back, but I forget what it was now. Either way, I have experienced the cold droplets hitting my hand the other day running a rebuilt SuperTigre G51 ringed engine.

Earl: Was that engine running WOT in both pics or idling?

earlwb 10-09-2012 03:33 AM

RE: Model Engine Spitting Fuel pics
The engine was idling at about 2500 RPMs. It was a crummy day, heavy overcast and the first cold front of the fall season.

Yeah they call it reversion and they use anti-reversion methods to reduce the effect.

There are two main causes of it. One is the air fuel column is being drawn into the engine crankcase and then the intake valve cuts it off and it doesn't want to stop due to inertia, so it sort of, bounces and goes out to some extent. This is similar to the water hammer effect seen with large water plumbing systems when valves open and close.

The second cause is the port timing and how long all of the ports are open in the engine. With more aggressive port timing, the ports can all be open momentarily (valves on a four stroke engine do it too). So that the exhaust port is open along with the bypass and transfer ports and intake port. This thus allows a pressure pulse to travel all the way back to the carburetor causing some fuel to be pushed back out. The intake port not being cut off early can also allow the crankcase pressure to start to build too. The engine designers time the engines like that so that at high speed there is a stuffing effect going on that helps ram more air/fuel into the crankcase. It is a noticeable performance boost with the engines. But at low speed the engine can spit a lot more fuel though.

I have a short video clip that shows the droplets getting sucked forward into the prop, but when I upload it, the video service seems to diddle with the clip and it loses the droplets being shown momentarily. So I can't post the video of it yet. But then it is one of those "blink" and you miss it kind of things anyway.

1QwkSport2.5r 10-09-2012 05:45 AM

RE: Model Engine Spitting Fuel pics
I've noticed some spitting from the carb with 2-strokes, especially when running in ringed or lapped engines when the mixture is super rich. My Enya 60-4C has a major spitting problem. The area behind the engine mount is always soaked with oil and fuel that spits out from the carb. It makes quite a mess. If this were in a plane, the airframe would be saturated by now.

Edit: changed some wording.

Sport_Pilot 10-09-2012 07:00 AM

RE: Model Engine Spitting Fuel pics
Cool, and usefull for more then demonstration.  Now you know how long a velocity stack has to be to get rid of the fuel standoff.

earlwb 10-09-2012 07:06 AM

RE: Model Engine Spitting Fuel pics
Usually one can minimze the spitting effect by using a intake stack to help contain it more. Saito has some intake stacks they use on some engines and provide it as a accessory option for other engines. The gas RC engine guys have used intake stacks for a long time on various gas engines too. I have noticed over the past few years where the control line engines mostly use a long intake venturi and stack on their engines too. Before then the control line engines all had really short intake to the venturi.

One other thought is that the carb will allow fuel to flow both ways, when air is passing by, so the spitting is really the reverse flow causing fuel to flow which shows up as the little droplets being expelled from the carb's mouth.

Here is some more information about it here http://www.circletrack.com/enginetec...ake_reversion/
Even though it is for four stroke car engines, pretty much the same thing happens with a two stroke engine too.

Sport_Pilot 10-09-2012 07:15 AM

RE: Model Engine Spitting Fuel pics

Actually its called fuel standoff, reversion is the reversal of intake and exhaust gases.Reversioncauses fuel standoff, but fuel stand off can be eliminated without reducing reversion. Automobiles have reversion, in fact all engines do,but the manifold, throttle body, breather, and filter work together to prevent fuel standoff. You can sometimes see the results of fuel stand off in an automobile in the form dirty air filters that pick up dirt from wet fuel and air. If the car is burning oil it will leave an oil stain on the air filter.</p>

Sport_Pilot 10-09-2012 07:21 AM

RE: Model Engine Spitting Fuel pics

Funny with the see-through prop too, sharp edges yet see-through.
But of course, it is a Glass prop!

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